observed an association of PM2.5 exposure levels with doctor
diagnosed asthma at the age of 6 years [26]. Finally, a
large Canadian study reported inconsistent results for the
associations of air pollutant levels with the risk of asthma
until the age of 4 years, depending on the exposure assessment.
The authors reported no association of traffic-related
air pollution based on land use regression modeling with
the risks of asthma, but reported associations of distance to
industrial point sources with an increased risk of asthma
[27]. Differences between our study and previous published
studies include our detailed method to assess air pollution
exposure levels in a large city, the availability of many